1. Field of the Invention
This invention is in the field of methods and apparatus for coating a substrate with a metal oxide coating by pyrolitic decomposition of a metal compound on the hot glass surface. More particularly, the invention is in the field of hoods for applying protective coating to hollow glass containers.
2. Description of the Prior Art
The desirability of applying protective coatings to glass in general and to the exterior of hollow glass containers in particular has long been recognized. Such coatings, which include those resulting from the application of tin, titanium and other metallic compounds, or other heat-decomposable organometallic compounds, protect the glass surface from damage such as from abrasion and scratches which cause a loss of tensile strength of the glass containers. The need for high tensile strength in, e.g., glass containers is particularly acute when the containers are mass-produced, move rapidly in close proximity along high-speed conveyor lines, and are subsequently filled with carbonated beverages, beer, wine, foodstuffs, and the like which can produce gaseous pressure within the container. Protective coatings are usually applied as the glass articles, generally containers, emerge in a heated, fully-shaped condition from a glassware-forming machine, that is, at the "hot" end of the system. The containers are thereafter transported away from the forming machine by a conveyor. Temperatures in excess of 400 degrees Centigrade (.degree.C.) exist at the surface of the glass containers, such that when a heat-decomposable inorganic metallic or organometallic compound is applied to those surfaces, the compound decomposes and is converted to a metallic-oxide coating.
One well-known and previously widely used technique for applying a protective coating to the hot glass containers calls for spraying the opposite sides of the containers as they travel on a conveyor in single file through spray heads positioned for optimal coating of the glass surface of a particular container. Receivers are positioned on the opposite side of the conveyor in alignment with the respective spray heads. Pressurized air or inert gas with the coating compound entrained therein is discharged from one or a plurality of spray heads at a significant positive pressure, while the receivers are usually maintained at a relatively low pressure. The resultant pressure differential increases the velocity, and thus the effectiveness, of the coating-precursor compound. Coating system of this nature are disclosed, inter alia, in U.S. Pat. No. 3,516,811, to Gatchet, et al., and U.S. Pat. No. 3,684,469, to Goetzer, et al.
Gatchet, et al. recognized that the deposition of a metallic-oxide coating on the finish of the glass container passing on a conveyor through the open-sided coating apparatus of the prior art was undesirable, as noted in column 3, lines 21-57 of U.S. Pat. No. 3,516,811. Gatchet, et al. relied upon spray heads producing a theoretically laminar flow which would pass laterally across the width of the conveyor to control the location as well as the uniformity of the metal-oxide deposit, as shown in FIG. 4 of that patent.
The above-described coating systems, however, are what may be termed "open-sided," and are thus adversely influenced by ambient conditions in the facility where the glass containers are formed. The ambient conditions of prime concern are rapidly-moving air currents, moisture in the air, and the potentially toxic and corrosive fumes and pollutants being discharged from the spray heads. Air currents can cause turbulent conditions at the spray heads, which can in turn result in a preferential or uneven application of the protective coating. Some of the coating will therefore accumulate on the bottle "finish", the term used in the industry to designate the closure region of the bottle. The rapidly-moving air currents disrupt the laminar-flow patterns which are theoretically possible with open-sided systems, and the capability for uniformly, and consistently, applying the same thickness of coating is seriously reduced.
To compensate for air currents as described above, the systems are therefore operated at higher pressures, and with the use of greater amounts of coating compound, than would be required under quiescent conditions. The necessary result of process adjustments such as these is the use of greater amounts of coating compound than required for optimum economy.
The mositure in the hostile atmosphere described above causes hydrolysis loss, thus rendering some of the compound unfit for its intended purpose. Further, the escape of potentially toxic fumes into the atmosphere at the work place can constitute an occupational health hazard, and may also be a violation of applicable law. These fumes are also generally quite corrosive, and can attack various components of the glass factory, such as, e.g., blowers, exhaust systems, conveyors and roofs, obviously leading to increased plant-maintenance costs. Additionally, the efficiency of these open-sided systems is low, since much of the relatively expensive coating compound is wasted.
A second, well-known, and widely employed technique for applying a protective coating to hot glass containers relies upon a formed sheet-metal coating hood with spray heads and associated receivers situated therein. The hood obviates many of the problems associated with the open-ended spray systems discussed above. For example, it isolates the glass containers from ambient conditions, and furnishes a controlled atmosphere which enhances the coating operations. The hood includes an exhaust system which captures most of the air-entrained coating compound not adhering to the containers, thus reducing the problem of venting the system and minimizing the opportunity for the coating compound to attack building components. Also, that hood can significantly raise the coating efficiency of the systems, with attendant cost savings.
Coating hoods substantially representative of the prior art are disclosed in U.S. Pat. No. 3,819,404 to Scholes et al.; U.S. Pat. No. 3,933,457, to Scholes; and U.S. Pat. No. 4,389,234 to Lindner. The most recent patent to Lindner, et al. presents a coating hood including a tunnel for allowing containers to pass therethrough, and a vertically adjustable flat roof for accommodating containers of various sizes. At least two jet slots are located in each side wall, and at least two receivers or suction slots are aligned therewith. The jet and suction slots are interspersed opposite each other in each sidewall. The coating compound is introduced through at least one feedpoint, and blowers secured to the sidewalls furnish an inner and an outer loop of high-velocity air, of which the inner loop contains the coating compound, to the interior of the hood. Baffles are situated in the flow path of the high-velocity air so that the jets issuing from the jet slots are well defined, and thus better suited for their intended function.